This is just a guess, but since the pet version of this request was declined, I’m assuming the item version would be declined as well. The reasoning is that there are even more items than pets, so adding all unowned items (or all items in general) to a wishlist would likely put an even greater strain on the server.
Here’s the
declined suggestion for reference: Add a button to the wishlist that allows you to 'add all pets missing' to the wishlist: searching through each users' missing pets would create too much strain on the server.